Nervous System- Class-XI

Nervous System

– It is Ectodermal in origin.
– It is the first system developed during embryogenesis.
– The structural and functional unit of nervous system is neuron or nerve cell. Human body contain 30 billion neurons.
– There are two systems which coordinate and regulate various organs of our body, these are
(i) Endocrine System (ii) Nervous System
– Endocrine system act as slow speed service. It secrete some chemical messangers (hormones) to regulate various body activities.
Nervous System –
– It act as high speed service or no delay service. It include components of three categories
Sensory organs (receptors), Communication system and Effectors (muscles or gland)
– The communication system involve central nervous system, peripheral nervous system and autonomous nervous system.
General Functions of Nervous System
– It receive the changes of external environment and respond them according past experience.
– It coordinates between various organs of body.
– It transmitt impulse to various parts.
– It establish the homeostasis in internal environment of body.
Nervous Tissue

Neurons Neuroglial cells

Neurons –
– It is largest cell of body. Each neuron has two parts-
1. Cell body or Cyton or Perikaryon or Soma
2. Cell processes : Dendrone and Axon
(1) Cell body
– It has a round and large nucleus. It also contain numerous neurofibrils, mitochondria, golgi complex and Nissl’s granules, ER, ribosomes and fat droplets.
– Nissl’s granules are also called Trigroid which are made up of RER + ribosome and they synthesise protein.
– Cytoplasm contain some small fibrills which are called neurofibrills. They helps in conduction inside cyton
– Neuron lacks centriole (Present in Neuroblast cells ) so they don’t divide after Embryonic Development. These cells remain in interphase throughout life.
– They have some power of regeneration.
– The cyton are generally found in Grey Matter of brain.

Note– The cyton of cerebellum are flask shaped which are called Purkinje cells
(2) Cell Processes:-
– Neuron cell give rise to various processes called Neurites which are of two types –
(i) Dendrites or Dendron
(ii) Axon or Axis cylinder
– Dendrites are small, numerous and conical shaped. They are thick at base and thin at apex. They contain all cell organelles along with Neurofibrils and Nissl’s granules.
– Dendrites tansmitt impulses from sensory cells or other neurons to the cyton so they are also called Afferent processes
– Axon are always one, long and of similar thickness.
– The terminal end of axon is branched having button shaped or knobbed structures called telodendria
– Telodendria is rich in mitochondria, which synthesise acetyl choline with the help of choline acetyl transferase.
– Axon has mitochondria and neurofibrills but lacks Nissl’s granules.
– Axon carry impulses away from cyton (towards other neuron or Effector organs). So axon is also called Efferent processes
– The cytoplasm of axon is called Axoplasm and its covering is called Axolemma.
– Axon is functional part of nerve cell.
Note– The nerve cell of female has bar body in its nucleus.

Nerve Fibre –
– Axon is surrounded by Neurilemma. The axon along with
Neurilemma is called Nerve fibre.
– The neurilemma is secreted by Schwann cells (multinucleated). This cell appear as a projection on this neurilemma.
– Around the neurilemma there is a thin basement membrane and outer to basement membrane there is a covering of connective tissue called Henle’s key Retzius Sheath.
– In long axon each Schwann cell coil around it to form another covering called Myelin sheath (medullary sheath).
– The myelin sheath is not a continuous covering. It is absent in nodes of Ranvier. These nodes are present at 1 mm distance.
– Myelin sheath lacks cytoplasm. It is made up of phospholipid and proteins of plasma membrane. The fat (lipid) is also called Myelin so the term is given Myelin sheath
Functions of Myelin Sheath
3It provides electrical insulation.
3It speed up impulse transmission (50 times).
3It helps in regeneration of neurone.
– Myelin sheath is not found in all neurons. The neuron having myelin sheath are called Myelinated (Medullated) neurone while the neuron lacking myelin sheath are called Non myelinated (non medullated)

Note– In CNS all neuron have Neurilemma but their neurelimma is not made up of schwann cells, it is formed by Neuroglial cells or Oligodendrocytes. These neuron also lack Henle’s sheath.
Note– Formation of myelin sheath is called myelinogenesis. In PNS it is carried out by schwann cells while in CNS it is carried out by oligodendrocytes (neuroglia).
Note– Myelin sheath act as heat insulator and inhibits the leakage of ions. It also conserve energy of axon.